Temperature compensated microwave cavity



United States Patent 3,138,240 TEMPERATURE COMPENSATED MECROWAVE CAVITYHenry J. Riblet, 35 Edmunds Road, Wellesley, Mass. Original applicationJan. 17, 1958, Ser. No. 709,598, new Patent No. 2,998,582, dated Aug.29, 1961. Divided and this application Aug. 28, 1961, Ser. No. 134,371

3 Claims. (Cl. 333-433) The present invention relates in general totemperature compensation and more particularly concerns means forstabilizing a frequency-sensitive dimension of a micro wave cavity in aprescribed manner over a wide temperature range. When employed as afrequency-controlling resonant cavity, the frequency-sensitive dimensionremains constant in the presence of temperature variations. As a result,a microwave signal characterized by exceptional frequency stability overa wide temperature range may be generated. The properties of such acavity are also advantageously employed in microwave circuits, such asfrequency discriminators, Where frequency response characteristicsindependent of temperature are desired. This application is a divisionalof my copending application Serial No. 709,598, filed January 17, 1958,now Patent No. 2,998,582.

The theory of temperature compensation of microwave cavities is fullydiscussed in Patent No. 2,790,151 entitled Temperature CompensatedCavity Resonator, issued April 23, 1957, to Henry I. Riblet. In thatpatent, there is disclosed a cavity resonator capable of yielding anexceptionally accurate frequency indication of a microwave signalcoupled thereto over a relatively broad frequency band and despiterelatively wide temperature variations. This was accomplished byselectively changing the temperature coefficient of expansion of themember controlling the cavity length as the cavity length changed fromone range to another.

The present invention contemplates and has as a primary object theextension of the range over which accurate temperature compensation of afrequency-sensitive dimension of a microwave cavity may be obtained.This is accomplished by controlling a frequency-sensitive dimension,generally length, of the cavity with different means over differenttemperature ranges, each of the means changing length by a differentincrement in response to an incremental temperature change of the samemagnitude. In one form of the invention, each means comprises amultiplicity of members of approximately the same length, but ofdifferent material having unlike temperature coefficients of expansion.Inanother form, each means comprises different lengths of the samematerial.

In a specific embodiment of the invention, the cavity is defined by agenerally hollow cylindrical conducting structure having a fixed endwall and an oppositely disposed axially movable confronting end wall. Aplurality of axially expansible sections are supported within thecylindrical structure. Resilient means urge the movable end wall and atleast one of the expansible sections in opposite axial directions. Thecylindrical structure and expansible sections have different temperaturecoefficients of expansion. As the tempera-ture varies, the fractionalchange in length in response to an incremental change in temperaturediffers for the cylindrical structure and each section. This differenceis utilized to cause a corresponding change in the axial force appliedto the movable end wall whereby it tends to move axially in a directionopposite to the change in length of the cylindrical structure. However,in any one temperature range, a selected one of said sections controlsthe magnitude of this restoring axial force applied to the end wall.

In one form, the sections comprise a plurality of con- 3,108,240Patented Oct. 22, 1963 cent-rically arranged annular cylinders havingdifferent temperature coefficients of expansion. In each temperaturerange, one of the annular cylinders is longer than the others and itsends separate a plug fixed in the end of the cylindrical structure froma slideable member for transmitting axial restoring forces to themovable end wall. Accordingly, expansion and contraction of the longestcylinder in a particular temperature range controls the magnitude of therestoring force.

In another form, the sections comprise different lengths of the samecylinder having an annular ridge adapted to fit loosely within anannular groove inside the cylindrical structure. In a first temperaturerange, the inside edge of the ridge, that is, the edge nearest themovable end wall, abuts the inside edge of the groove and the restoringforce is determined by length changes of the section of the cylinderbetween its inside end and the inside edge of the ridge. In a secondtemperature range, the ridge contacts neither groove and length changesof the entire cylinder governs the magnitude of the restoring force. Ina third temperature range, the outside edges of the ridge and groovemeet and length changes of the cylinder section from the outside ridgeedge to its outside end controls the restoring force.

Other objects, features, and advantages of the invention will becomeapparent from the following specification 'when read in connection withthe accompanying drawing in which:

FIG. 1 shows an embodiment of the invention wherein the length adjustingsections comprise a pair of concentrically arranged annular cylindershaving different temperature coefiicients of expansion;

FIG. 2 illustrates an alternative form of the invention with the lengthadjusting member comprising a cylinder having an annular ridge looselyengaging an annular ridge inside the generally hollow cylindricalstructure; and

FIG. 3 shows an embodiment of the invention,

especially suitable for large scale production because of the relativeease fabrication.

Similar elements are designated by the same reference symbol throughoutthe drawing.

With reference now to the drawingand more particularly FIG. 1 thereof, afirst form or" the temperature compensated microwave cavity isillustrated. The cavity 11 is defined by the generally hollowcylindrical structure 12, shown in cross-section, having a fixed endwall 13 and a movable end wall formed of piston 14 opposite the latterand free to slide axially within cylindrical structure 12. Piston 14 isformed with a central collar section 15 and neck section 16 outside thecavity 11. A resilient diaphragm 17 surrounds neck section 16, is insurface contact with collar section 15, and is secured at its outerperiphery by welding or other suitable means to the wall of cylindricalstructure 12 whereby an axial force is imparted upon piston 14 urging itto the left.

Neck section 16 is integral with or rigidly attached to a bolt 21 at theflat disc-shaped head 22 thereof. The stem 23 of bolt 21 is centrallysupported within end plug 24 in a manner permitting the bolt 21 to slideaxially but not radially. End plug 24 is screwed into the right end ofcylindrical structure 12 after the remaining internal elements have beeninserted.

A threaded ring 25 is screwed inside cylindrical structure 12 and holdsdiaphragm 17 firmly in place. A resilient annular disc 26 is lockedbetween ring 25 and a threaded cylindrical shell 27. The radially inwardedge of disc 26 rests in shoulder 31 of head 22 and urges bolt 21 to theright.

Annular cylinders 32 and 33 coaxially surround stem 23. The edges of atleast one of these cylinders, depending on the temperature, contact head22 and end plug 24.

Having described the structural arrangement of a temperature compensatedmicrowave cavity, its mode of operation will be discussed. Preferably,all the rigid structural elements except annular cylinders 32 and 33 aremade of Invar having a low temperature coeihcient of expansion. Cylinder32 is made of Invar having a higher temperature coeflicient of expansionthan cylindrical structure 12 and cylinder 33 is formed of brass havinga still higher temperature coefficient of expansion. Thus, as thetemperature changes, the fractional change in length of cylindricalstructure 12 and the other elements made of like Invar, of Invarcylinder 32, and of brass cylinder 33, diifers.

If no temperature compensation were provided, a change in temperaturewould be accompanied 'by a change in the length L of cavity 11 as aresult of contraction or expansion of cylindrical structure 12. Inaccordance with the inventive concepts, piston 14 is moved axially in adirection tending to counteract the change in length of cylindricalstructure 12 and the cavity length L -accordingly remains substantiallyconstant, regardless of tem-.

perature. In a first temperature range, a corrective displacement issupplied by changes in length of cylinder 32 while in a second rangesuch corrections are in response to changes in length of cylinder 33.This will be better understood by considering the following example.

At a specified temperature, typically centigrade, cylinders 32 and 33are of the same length and in edge contact with head 22 and end plug 24.Since Invar cylinder 32 has a lower temperature coefficient of expansionthan brass cylinder 23, the fractional change in length of the formerfor a given change in temperature is less than that of the latter.Accordingly, at temperatures below 0 centigrade, only cylinder 32 is inedge contact with head 22 and end plug 24. As a result, below thistemperature, contractions and expansions thereof are transmitted throughhead 22 to piston 14 rigidly connected thereto and move the pistonaxially to compensate for changes in length of cylindrical structure 12due to temperature variations in this region. Above 0 centigrade,compensatory movement occurs in response to length changes of brasscylinder 33.

Resilient diaphragm 17 and resilient annular disc 26 center the rigidlyconnected piston 14 and bolt 21 and impart axial forces thereon inopposite directions to aid in overcoming any static friction betweenslidable elements and cylindrical structure 12. Thus, slightcompensatory changes in length are immediately followed.

With reference to FIG. 2, there is illustrated an alternative embodimentof the invention. Elements corresponding substantially to those in FIG.1 are identified by like reference numerals. Cavity 11 is defined by thegenerally hollow cylindrical structure 12 having a fixed end wall 13 andaccommodating piston 14-, the axially slideable end wall of the cavity11. Resilient diaphragm 17 surrounds neck section 16, is in surfacecontact with collar section 15, and is secured at its outer periphery bywelding or other suitable means to the wall of cylindrical structure 12whereby an axial force is exerted upon piston 14 urging it to the left.Cylindrical structure 12 and neck section 16 have annular ridges 34 and35, respectively, extending radially inward and outward respectively. Aresilient diaphragm 36 surrounds neck section 16 and presses against theright edge of ridge 34 and the left edge of ridge 35 at its outer andinner peripheries, respectively, thereby urging piston 14 to the right.

In the right section of cylindrical structure 12, there is an annulargroove 38 extending radially outward from the inner surface ofcylindrical structure 12. A brass cylinder 39 is supported within theright end of cylindrical structure 12 and has as an annular ridge 41extending radially outward adapted for loose mating relationship withannular groove 38. At the left end, cylinder 39 has an 'axially alignedconical groove 42 separated from a slrnil'arly aligned conical groove 43by ball 40. The right end of cylinder 39 also has an axially alignedconical groove 48 facing a similar groove 44 in cup-shaped end cap 45and separated therefrom by ball 46. The inner tangential edge 47 of endcap 45 slides over the annular ridge 51 of cylindrical structure 12. Aresilient spring 52 between end cap 45 and the radially inward extendingannular ridge 53 at the right end of cylindrical structure 12 exerts aforce toward the left on cylinder 39 through ball 46 and end cap 45, andalso serves to keep the latter elements in place.

Temperature compensation is obtained in this embodiment in the fOllOWingmanner. Initially, it is convenient to assume that the temperature is inthe lowest range and brass cylinder 39 contracted. At this time, theforce exerted by resilient spring 52 toward the left is suflicient toovercome oppositely directed forces from other resilient members andforce cylinder 39 to the left until the left or inner edge of annularridge 41 contacts the corresponding edge 54 of groove 38. Compensationin this temperature range is then effected by expansion and contractionof the left section of length A between the left or inner end ofcylinder 39 and the left or inner edge 55.

In the next higher temperature range, the force exerted by resilientdiaphragm 36 to the right overcomes other oppositely directed forces andthe left edge 55 moves away from left edge 54. In this range, neitheredge of annular ridge 41 contacts a radial edge of groove 38 and lengthchanges of the entire cylinder length B govern th correctivedisplacement imparted to piston 14.

Finally, the highest temperature range is reached when cylinder 39 hasmoved toward the right with right edge 57 of annular ridge 41 contactingthe corresponding edge 56 of annular groove 38. Compensation is thendetermined by expansion and contraction of the section of length Cextending from the left end of cylinder 39 to the right or outer edge57.

Since the incremental change in length is given by:

AL atLAT where L is the length of the expansible section, at is itsthermal coefiicient of expansion, and AT is the incremental change intemperature, in the lowest range, the compensating length change isAL=uAAT In the intermediate temperature range, half the incrementallength change of the cylinder is imparted to the left to correct theposition of piston 14 and the other half moves resilient spring 52 tothe right. Accordingly, in this range, the compensating length change isAL 012A T Finally, in the third range the compensating length change isAL=0CAT Cylinder 39 may be constructed of sections of material havingdifferent thermal coefiicients of expansion bolted together to obtaindifferent degrees of compensation. There may be several annular ridgesand mating grooves. Various different combinations of spring tensionsmay be employed.

With reference to FIG. 3, there is illustrated still another embodimentof the invention for obtaining precise temperature compensation. Theadvantages of the other embodiments are retained, yet fabrication inlarge quantities is made still easier. The axially movable end wall is aresilient conducting disc 61. The peripheral edges of disc 61 and piston14 are braised together at 69. A hollow unitary structure 62 is made ofInvar having a temperature coefficient of expansion different from theInvar used for the cylindrical housing 12. The unitary structure 62 isformed with a stem 63 and a hub 64, all coaxially surrounding a brassrod 65. The right ends of rod 65 and stem 63 are soldered together. Theremaining portion of rod 65 is free to expand and contract within thehollow chamber 66. A resilient diaphragm 17 is welded to cylinder 12 andstem 63 at its outer and inner circumferential edges, respectively. Theright end of stem 63 seats in the mating opening within end plug 24. InFIG. 3, stem 63 is shown having a shoulder 67 held in abuttingrelationship with end plug 24 by the action of resilient diaphragm 17.Thus, the cavity length L may be initially adjusted by rotating end plug24 to impart axial motion to structure 62 without rotation.Alternatively, where no adjustment of cavity length is desired, stem 63may be rigidly secured to end plug 24 by soldering or other suitablemeans.

Over the low temperature range, disk 61 remains in contact with hub 64by virtue of its own resiliency, and changes of length of structure 62determine the compensation for temperature variations. In the hightemperature range, the left end of brass rod 65 extends beyond thesurface of hub 64 to exert an axial force on disk 61 to cause atemperature compensating displacement thereof.

There has been described a temperature compensated microwave cavitycapable of retaining a desired dimension over a wide temperature range.It is apparent that those skilled in the art may now make numerousmodifications of and departures from the specific embodiments describedherein without departing from the inventive concepts. Consequently, theinvention is to be construed as limited only by the spirit and scope ofthe appended claims.

What is claimed is:

1. A temperature compensated microwave cavity resonator comprising, agenerally hollow cylindrical conducting structure having a fixed endwall and an oppositely disposed axially movable confronting end wall, anexpansible axially movable plug disposed adjacent said movable end walland externally of the cavity resonator defined between said fixed andmovable end walls, means for urging one end of said plug and saidmovable end wall into engagement, and cooperating means on said plug andon said cylindrical structure for fixing a different expansible axiallength of said plug for controlling the axial position of said movableend wall in each of a plurality of contiguous temperature ranges.

2. A temperature compensated microwave cavity resonator comprising, agenerally hollow cylindrical conducting structure having a fixed endwall, an oppositely disposed axially movable confronting end wall, saidhollow cylindrical conducting structure being formed with an internalannular groove in a region axially displaced from the cavity resonatordefined between said fixed and movable end walls, an expansiblecylindrical plug within said cylindrical structure and formed with anannular ridge disposed between the ends of said plug and fitted in loosemating relationship within said annular groove, means urging saidmovable end wall into contact with the adjacent end of said cylindricalplug and means urging said cylindrical plug toward said movable end wallwhereby over a plurality of contiguous temperature ranges said annularridge assumes a respective plurality of axial positions within saidannular groove; a difierent length of said expansible cylindrical plugbeing thereby determinative of the position of said movable end wall ineach of said plurality of temperature ranges.

3. A temperature compensated microwave cavity resonator comprising, agenerally hollow cylindrical structure having a fixed end wall, supportmeans affixed to said cylindrical structure at the end opposite saidfixed end Wall, a hollow cylindrical expansible member extending axiallyinward from said support means toward said fixed end wall and formed atits inner end with an outstanding central hub and an outstandingshoulder conforming generally with the internal dimension of said hollowcylindrical structure, a flexible conducting member rigidly secured atits edge to said shoulder and normally urged toward and contacting saidhub at its center, the inner confronting faces of said flexible memberand said fixed end wall defining said cavity resonator between them, andan axially expansible rod of different temperature coefficient ofexpansion extending axially through said hollow cylindrical expansiblemember and fixed at one end relative to said support means, the otherend of said rod extending through said hub into contacting relation withsaid flexible conducting member, whereby in one temperature range theaxial dimension of said hollow cylindrical expansible member solelycontrols the relative axial position of said flexible member while in asecond contiguous temperature range said hollow cylindrical expansiblemember and said expansible rod jointly control the relative axialposition of said flexible member.

No references cited.

1. A TEMPERATURE COMPENSATED MICROWAVE CAVITY RESONATOR COMPRISING, AGENERALLY HOLLOW CYLINDRICAL CONDUCTING STRUCTURE HAVING A FIXED ENDWALL AND AN OPPOSITELY DISPOSED AXIALLY MOVABLE CONFRONTING END WALL, ANEXPANSIBLE AXIALLY MOVABLE PLUG DISPOSED ADJACENT SAID MOVABLE END WALLAND EXTERNALLY OF THE CAVITY RESONATOR DEFINED BETWEEN SAID FIXED ANDMOVABLE END WALLS, MEANS FOR URGING ONE END OF SAID PLUG AND SAIDMOVABLE END WALL INTO ENGAGEMENT, AND COOPERATING MEANS ON SAID PLUG ANDON SAID CYLINDRICAL STRUCTURE FOR FIXING A DIFFERENT EXPANSIBLE AXIALLENGTH OF SAID PLUG FOR CONTROLLING THE AXIAL POSITION OF SAID MOVABLEEND WALL IN EACH OF A PLURALITY OF CONTIGUOUS TEMPERATURE RANGES.
 3. ATEMPERATURE COMPENSATED MICROWAVE CAVITY RESONATOR COMPRISING, AGENERALLY HOLLOW CYLINDRICAL STRUCTURE HAVING A FIXED END WALL, SUPPORTMEANS AFFIXED TO SAID CYLINDRICAL STRUCTURE AT THE END OPPOSITE SAIDFIXED END WALL, A HOLLOW CYLINDRICAL EXPANSIBLE MEMBER EXTENDING AXIALLYINWARD FROM SAID SUPPORT MEANS TOWARD SAID FIXED END WALL AND FORMED ATITS INNER END WITH AN OUTSTANDING CENTRAL HUB AND A OUTSTANDING SHOULDERCONFRONTING GENERALLY WITH THE INTERNAL DIMENSION OF SAID HOLLOWCYLINDRICAL STRUCTURE, A FLEXIBLE CONDUCTING MEMBER RIGIDLY SECURED ATITS EDGE TO SAID SHOULDER AND NORMALLY URGED TOWARD AND CONTACTING SAIDHUB AT ITS CENTER, THE INNER CONFRONTING FACES OF SAID FLEXIBLE MEMBERAND SAID FIXED END WALL DEFINING SAID CAVITY RESONATOR BETWEEN THEM, ANDAN AXIALLY EXPANSIBLE ROD OF DIFFERENT TEMPERATURE COEFFICIENT OFEXPANSION EXTENDING AXIALLY THROUGH SAID HOLLOW CYLINDRICAL EXPANSIBLEMEMBER AND FIXED AT ONE END RELATIVE TO SAID SUPPORT MEANS, THE OTHEREND OF SAID ROD EXTENDING THROUGH SAIDHUB INTO CONTAINING RELATION WITHSAID FLEXIBLE CONDUCTING MEMBER, WHEREBY IN ONE TEMPERATURE RANGE THEAXIAL DIMENSION OF SAID HOLLOW CYLINDRICAL EXPANSIBLE MEMBER SOLELYCONTROLS THE RELATIVE AXIAL POSITION OF SAID FLEXIBLE MEMBER WHILE IN ASECOND CONTIGUOUS TEMPERATURE RANGE SAID HOLLOW CYLINDRICAL EXPANSIBLEMEMBER AND SAID EXPANSIBLE ROD JOINTLY CONTROL THE RELATIVE AXIALPOSITION OF SAID FLEXIBLE MEMBER.